Inkjet recording methods have advantages over other image recording methods such that the process is simple; full color images can be produced relatively easily; and high resolution images can be produced even when an inkjet recording apparatus having a simple structure is used. Dye-based inks, in which a water soluble dye is dissolved in a medium such as water and mixture solvents of water and a wetting agent, have been typically used as inkjet inks.
Recently, there is an increasing need for high speed inkjet recording. When images are formed on a plain paper using an aqueous ink, serious problems such as a curling problem in that the resultant print is curled, and a cockling problem in that the resultant print is waved tend to be caused. This is because such an aqueous ink includes a large amount of water, and therefore water in the ink swells the fibers of recording papers, thereby often causing the curling problem and the cockling problem.
In contrast, oil-based inks do not include water, and therefore oil-based inks do not cause the curling problem and the cockling problem. However, oil-based inks have a greater penetration ability than aqueous inks, and therefore oil-based inks often cause serious problems concerning image qualities such that the resultant print has low image density; blurred images are formed; and an image penetrates deeply so as to reach the backside of a recording paper, thereby soiling the backside of the recording paper (hereinafter referred to as a backside soiling problem).
Namely, there is no inkjet ink which can produce images having good image qualities without causing the curling and cockling problems.
In attempting to solve the curling and cockling problems using an aqueous ink and a plain paper, a technique is proposed which uses, as a curl preventing agent, a specific compound such as saccharides, sugar alcohols, and amide compounds. However, when an image including a large amount of ink is formed at a high speed, the curling and cockling problems tend to be caused.
In addition, in attempting to solve the curling and cockling problems, JP-2005-297549-A discloses a method using an aqueous ink which includes a water-soluble organic compound in a relatively large amount. As a result of the present inventors' investigation, the ink disclosed in JP-2005-297549-A includes a water-soluble organic compound (solvent) in a relatively large amount, and therefor the low image density problem is caused like oil-based inks. In addition, the ink has a high viscosity, and therefore the ink has poor ejection property (i.e., the ejection reliability of the ink deteriorates).
In addition, in attempting to solve the curling and cockling problems, a technique of using a hydrophobic solvent in a large amount is studied. However, the curling and cockling preventing effect of the technique is not satisfactory.
Thus, there is no inkjet ink which can produce images having good image qualities without causing the curling and cockling problems.
JP-2012-107210-A proposes an ink including a polyalcohol having an equilibrium moisture content of not less than 30% by weight at 23° C. and 80% RH, and a specific amide compound (such as 1,3-dimethyl-2-imidazolidinone, β-methoxy-N,N-dimethylpropion amide, and 2-pyrrolidone). Using such an amide compound produces the curl preventing effect.
JP-2011-201063-A proposes a clear ink including a glycol ether (such as diisoprene glycol monoalkyl ether, and triisoprene glycol monoalkyl ether) in an amount of from 3 to 10% by weight. The ink does not include a colorant, and includes a water-soluble organic compound in a relatively small amount.